The present invention relates to an ink composition for an electrostatic inkjet system by which the ink composition can be ejected by utilizing an electrostatic field, and the present invention also relates to an inkjet recording method using the ink composition.
For electrostatic inkjet recording, an ink composition (hereinafter, referred to as ink) prepared by dispersing charged color fine particles in a dispersion medium is used and an image corresponding to image data is recorded on a recording medium by applying a predetermined voltage depending on the image data to each ejecting portion formed in an inkjet head to eject and control ink using electrostatic force.
Examples of an electrostatic inkjet recording apparatus known in the art include one disclosed in JP 10-138493 A.
FIG. 4 shows a schematic diagram of an inkjet head of the electrostatic inkjet recording apparatus disclosed in JP 10-138493 A.
An inkjet head 80 as shown includes a head substrate 82, an ink guide 84, an insulating substrate 86, a control electrode 88, an opposing electrode 90, a DC bias supply 92, and a pulse power supply 94.
Nozzles (through-holes) 96 for ejecting ink are formed in the insulating substrate 86. In addition, the head substrate 82 formed extends along the array of those nozzles 96. On the head substrate 82, each of the ink guides 84 is arranged at a position corresponding to the through-hole. The ink guide 84 extends through the nozzle 96 and its tip portion 84a protrudes from the surface of the insulating substrate 86 facing to a recording medium P.
The head substrate 82 and the insulating substrate 86 are arranged at a predetermined distance from each other and also a flow path 98 for ink I is formed between them.
The ink I including colorant-containing particles (colorant particles) which are charged with the same polarity as that of a voltage applied to the control electrode 88 is circulated in the ink flow path 98 from the right side to the left side in FIG. 4 by means of an ink circulating mechanism (not shown) to be supplied to each nozzle 96.
The control electrode 88 is formed in the shape of a ring and placed on the surface of the insulating substrate 86 facing to the recording medium P such that the nozzle 96 is surrounded with the control electrode 88. In addition, the control electrode 88 is connected to the pulse power supply 94 that generates a pulse voltage depending on image data, and also the pulse power supply 94 is grounded via the DC bias supply 92.
The recording medium P is held on an insulating layer 91 of the grounded opposing electrode 90 while being charged to a high reverse voltage with respect to the control electrode by means of a charging device such as a scorotron charging device. Therefore, in this system, the recording medium P acts as an opposing electrode and the high voltage of the recording medium P acts as a bias voltage.
In the electrostatic inkjet recording described above, when no voltage is applied to the control electrode 88, the Coulomb attraction between the bias voltage of the opposing electrode 90 and the colorant particles (charged particles) in ink, the viscosity and surface tension of the ink (dispersion medium), the repulsive force between the colorant particles, the fluid pressure of the ink supplied, and so on are combined and balanced in the form of meniscus as shown in FIG. 4 in which the ink I rises a little from the nozzle 96.
In addition, those factors including the Coulomb attraction allow the colorant particles to migrate to the meniscus. In other words, the ink I is concentrated.
When a voltage is applied to the control electrode 88, the drive voltage is superimposed on the bias voltage and as a result the ink I is sucked toward the recording medium P (opposing electrode), forming an ink liquid column in the shape of a generally cone, the so-called Taylor cone.
After the Taylor cone formation, as time passes further, the Coulomb attraction acting on the colorant particles and the surface tension of the dispersion medium become unbalanced, resulting in a spindly ink liquid column of a few micrometers to several tens of micrometers in diameter, which may be referred to as a thread because of thread-forming property (spinnability). After that, as time passes moreover, the tip of the thread becomes successively disrupted to eject liquid droplets of the ink I. The flying of the ink droplet R is also facilitated with the action of suction force caused by the bias voltage. Consequently, the ink droplet R reaches the recording medium P.
In the electrostatic inkjet recording, generally, an ejection voltage is applied to each control electrode 88 so that the control electrode 88 can be switched on/off to modulate the ink droplet R before ejection. Therefore, the on-demand ejection of the ink droplet R can be carried out depending on an image to be recorded.
Here, the frequency of disrupting the thread is far higher than the frequency of voltage application (the frequency of driving the control electrode 88 (pulse power supply 94)) required for ejecting the ink droplet R corresponding to a dot. Within the time period of a single application of the driving voltage, the thread is successively disrupted many times. In other words, a dot can be formed on the recording medium by multiple fine liquid droplets caused by the disruption of the thread. The electrostatic inkjet recording utilizes such a phenomenon to control the time period of applying a voltage (the so-called “pulse width”) for the formation of a dot. Therefore, an improvement in uniformity of dot diameter on the recording medium P can be attained by adjusting the volume of the liquid droplet (or the number of the liquid droplets) for the formation of a dot. In addition, high gradation of image recording can be attained by carrying out the control of density gradation or the like by means of intentional adjustment of the dot diameter.
Although the electrostatic inkjet recording has such excellent characteristic features, the so-called “ejection delay” occurs as the need of a slight time lag from the start of voltage application to the control electrode 88 to the start of the ejection of a liquid droplet. The ejection delay serves as a cause of lowering controllability and so on.
More specifically, in the electrostatic inkjet recording, as described above, the ink composition forms the Taylor cone after the application of a voltage to the control electrode 88, a thread is then formed and grown, and the tip of the thread is disrupted to eject an ink droplet. The ejection delay is caused by the steps from the formation/growth to disruption of the thread.
In the electrostatic inkjet recording, the ejection delay serves as a cause of a decrease in image quality or a decrease in frequency responsivity by variations in dot diameter, i.e., a cause of preventing productivity.